These are the commercial jet engines you see, like the Boeing 777, 747, Airbus A330 and A380s. Though there is a considerable reduction of the engine thrust, at these altitudes the lower drag associated with low air density means a more economical flight with a higher True Air Speed.
Crepitus in the knees may be caused by knee injuries such as meniscus tears. These are fairly common in people who play sports, jog, or run. A meniscus tear can cause crepitus as the joint moves. Chondromalacia patella is a dull ache behind the kneecap, usually caused by overuse or injury. There will be damage to the undersurface cartilage covering the kneecap.
These symptoms will change from week to week with intermittent diarrhea and then constipation. The syndrome is more chronic than just a few days, so your symptoms may just be due to gastrointestinal virus. You will have to see how quickly you get relief. Very severe constipation can also cause loose stools. If stools are hard and are not moving, they cause back up of more watery contents behind them. Eventually the watery contents spill around the sides of the hard constipated stool and cause sudden diarrhea. This is another instance when constipation and diarrhea can seem to appear together.